It is common practice to treat many types of ailments by implanting medical leads within a body. For example, if a person's heart does not beat properly, medical electrical leads may be positioned within the heart or surrounding vessels to deliver electrical stimulation to the heart.
One problem with positioning leads within the body relates to maintaining the leads at a desired location. When a lead is used to deliver an electrical signal to the heart, for example, it is important to maintain the device at a desired position so that the signal will cause desired electrical depolarization of the tissue. If the device shifts position, this desired depolarization may not occur.
The foregoing problem has been addressed by providing a variety of mechanisms for affixing leads to tissue. For example, passive mechanisms such as fixation tines are included at the distal ends of some pacing leads. These types of mechanisms are adapted to engage the trebeculae of the heart so that the lead is fixed in place until tissue begins to grow to further stabilize the device. Other types of active fixation mechanisms such as helical screws may be used to penetrate the tissue to maintain lead positioning.
While the foregoing fixation mechanisms are adequate for affixing a lead within a heart chamber, these methods are not particularly suited for maintaining the position of a lead within a vessel. Although tines may be used to lodge a lead within a vessel, the tines may undesirably block a substantial portion of the blood flow through the vessel. In other cases wherein leads are positioned within large vessels such as the coronary sinus, the tines are often not large enough to extend to the vessel walls to accomplish fixation. Similar problems exist with active fixation mechanisms. For example, a typical helical screw is positioned at the end of a lead such that it is substantially parallel to the walls of a vessel into which the lead has been inserted. This makes fixation using the helix difficult.
One alternative fixation mechanism involves providing an expandable balloon-like structure at the distal end of a lead. When in the expanded state, the balloon contacts the surrounding walls of a vessel to hold the lead in position. This method of fixation occludes the vessel, however, and is therefore not desirable for use in chronic implant situations.
Some of the foregoing problems have been addressed by U.S. Pat. No. 5,954,761 to Machek et al., which discloses a lead assembly that uses one or more stents that expand to contact an inner surface of a body vessel. Once deployed, this type of mechanism may not be retracted, as may be necessary to move the lead to another implant site. Additionally, this type of mechanism makes chronic lead removal difficult, if not impossible, since the stent will become attached to surrounding tissue over time. Finally, the disclosed stent may not be deployed a selectable amount to accommodate vessels of varying sizes.
What is needed, therefore, is an improved system and method for affixing leads within a body that addresses the foregoing problems.